实体框架5 MaxLength

Question

我使用EF4和我找到的一段代码从如下实体获取MaxLength值：

public static int? GetMaxLength(string entityTypeName, string columnName)
        {
            int? result = null;
            using (fooEntities context = new fooEntities())
            {
                Type entType = Type.GetType(entityTypeName);
                var q = from meta in context.MetadataWorkspace.GetItems(DataSpace.CSpace)
                                  .Where(m => m.BuiltInTypeKind == BuiltInTypeKind.EntityType)
                        from p in (meta as EntityType).Properties
                        .Where(p => p.Name == columnName
                                    && p.TypeUsage.EdmType.Name == "String")
                        select p;

                var queryResult = q.Where(p =>
                {
                    bool match = p.DeclaringType.Name == entityTypeName;
                    if (!match && entType != null)
                    {
                        match = entType.Name == p.DeclaringType.Name;
                    }

                    return match;

                }).Select(sel => sel.TypeUsage.Facets["MaxLength"].Value);
                if (queryResult.Any())
                {
                    result = Convert.ToInt32(queryResult.First());
                }

                return result;
            }
        }

然而，我升级到了EF5，我得到了这个错误消息：

...fooEntities'  does not contain a definition for 'MetadataWorkspace' and no
extension method 'MetadataWorkspace' accepting a first argument of type
'...fooEntities' could be found (are you missing a using directive or an assembly
 reference?)

从EF5获取元数据的最佳方法是什么？

Answer 1

这意味着您不仅升级了EF，而且还更改了API。有两个接口--核心ObjectContext接口和简化的DbContext接口。您的代码依赖于ObjectContext API (EF4中唯一可用的API )，但EF5使用EF4.1之后添加到单独的EntityFramework.dll程序集中的EF5。如果你想使用新的EF特性和以前的代码，你应该升级到.NET 4.5。

如果你还想使用一个新的API，你将不得不更新很多现有的代码，但是仍然可以从DbContext获取ObjectContext并使你的方法再次工作。您只需要使用以下代码片段：

var objectContext = ((IObjectContextAdapter)context).ObjectContext;

并且在代码中使用objectContext而不是context。

Answer 2

这是一段非常方便的代码。我对它进行了一些重构，它非常有用，我想我应该把它贴在这里。

public static int? GetMaxLength<T>(Expression<Func<T, string>> column)
    {
        int? result = null;
        using (var context = new EfContext())
        {
            var entType = typeof(T);
            var columnName = ((MemberExpression) column.Body).Member.Name;

            var objectContext = ((IObjectContextAdapter) context).ObjectContext;
            var test = objectContext.MetadataWorkspace.GetItems(DataSpace.CSpace);

            if(test == null)
                return null;

            var q = test
                .Where(m => m.BuiltInTypeKind == BuiltInTypeKind.EntityType)
                .SelectMany(meta => ((EntityType) meta).Properties
                .Where(p => p.Name == columnName && p.TypeUsage.EdmType.Name == "String"));

            var queryResult = q.Where(p =>
                                          {
                                              var match = p.DeclaringType.Name == entType.Name;
                                              if (!match)
                                                  match = entType.Name == p.DeclaringType.Name;

                                              return match;

                                          })
                .Select(sel => sel.TypeUsage.Facets["MaxLength"].Value)
                .ToList();

            if (queryResult.Any())
                result = Convert.ToInt32(queryResult.First());

            return result;
        }
    }

你可以这样叫它：

GetMaxLength<Customer>(x => x.CustomerName);

这里假设您已经在类型为Customer的DbContext中定义了一个DbSet，它具有一个定义了MaxLength的CustomerName属性。

这对于创建Model属性这样的事情非常有用，这些属性将textbox的maxlength设置为数据库中字段的最大长度，始终确保两者相同。

Answer 3

我将mccov002的示例重构为一个可复制-粘贴的扩展方法类：

using System.Data.Entity;
using System.Data.Entity.Infrastructure;
using System.Data.Metadata.Edm;

public static class DbContextExtensions
{

    // get MaxLength as an extension method to the DbContext
    public static int? GetMaxLength<T>(this DbContext context, Expression<Func<T, string>> column)
    {
        return (int?)context.GetFacets<T>(column)["MaxLength"].Value;
    }

    // get MaxLength as an extension method to the Facets (I think the extension belongs here)
    public static int? GetMaxLength(this ReadOnlyMetadataCollection<Facet> facets)
    {
        return (int?)facets["MaxLength"].Value;            
    }

    // just for fun: get all the facet values as a Dictionary 
    public static Dictionary<string,object> AsDictionary(this ReadOnlyMetadataCollection<Facet> facets) {
        return facets.ToDictionary(o=>o.Name,o=>o.Value);
    }


    public static ReadOnlyMetadataCollection<Facet> GetFacets<T>(this DbContext context, Expression<Func<T, string>> column)
    {
        ReadOnlyMetadataCollection<Facet> result = null;

        var entType = typeof(T);
        var columnName = ((MemberExpression)column.Body).Member.Name;

        var objectContext = ((IObjectContextAdapter)context).ObjectContext;
        var test = objectContext.MetadataWorkspace.GetItems(DataSpace.CSpace);

        if (test == null)
            return null;

        var q = test
            .Where(m => m.BuiltInTypeKind == BuiltInTypeKind.EntityType)
            .SelectMany(meta => ((EntityType)meta).Properties
            .Where(p => p.Name == columnName && p.TypeUsage.EdmType.Name == "String"));

        var queryResult = q.Where(p =>
        {
            var match = p.DeclaringType.Name == entType.Name;
            if (!match)
                match = entType.Name == p.DeclaringType.Name;

            return match;

        })
            .Select(sel => sel)
            .FirstOrDefault();

        result = queryResult.TypeUsage.Facets;

        return result;

    }

}